Research Article Cite This: ACS Catal. 2017, 7, 7509-7519
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ZIF-67-Derived Nanoreactors for Controlling Product Selectivity in CO2 Hydrogenation Guowu Zhan and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 Cambridge Centre for Advanced Research in Energy Efficiency in Singapore, 1 Create Way, Singapore 138602
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S Supporting Information *
ABSTRACT: CO2 hydrogenation to produce useful C1 chemicals (such as CO, CH4, and CH3OH) plays a pivotal role in future energy conversion and storage, in which catalysts lie at the heart. However, our fundamental understanding of the correlation between catalyst structures and product selectivity is still limited because in most cases the catalyst structures in nanoscale are not well-defined. Herein, we report the design and synthesis of nanoreactors by phase transformations of sandwich-structured ZIF-67@Pt@mSiO2 nanocubes via a simple water-soaking method where ZIF-67 serves not only as a morphological template but also as a sacrificial cobalt source. The resultant porous mazelike nanoreactors are highly active in gas-phase CO2 hydrogenation, in which the reaction pathway involves (i) dissociation of CO2 to form CO over Pt site via reverse water−gas shift reaction and then (ii) methanation of CO catalyzed by the nearby cobalt site. It was found that the overall “long retention time” for feed gases on catalysts significantly affected the product distribution. Thus, the specific activity (in the form of turnover frequency) of the nanoreactor having prolonged diffusion paths was around six times as much as that of other comparative catalysts with shorter diffusion paths. This work contributes insights to the CO2 hydrogenation to methane over bifunctional nanoreactors with designed structures. KEYWORDS: metal−organic frameworks, CO2 hydrogenation, catalysis, diffusion pathway, nanoreactors
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INTRODUCTION Utilization of carbon dioxide (CO2) via catalytic hydrogenation processes to produce useful fuels and chemicals (e.g., CO, CH4, olefin, methanol, formic acid, dimethyl ether, etc.) has been extensively studied in recent years due to both economic benefits and environmental concerns (fixation of “greenhouse” gas).1,2 An effective catalyst is essential to activate and split the thermodynamically stable CO2 molecule, and exploring new catalysts with aimed activity, selectivity, and prolonged lifetime is still necessary to industrialize the process. Regarding the kinetics and mechanisms (e.g., intermediates, elementary steps, active sites, etc.), however, there is still no consensus, and some existing mechanisms are even highly debatable. For instance, there are two totally different reaction mechanisms proposed for CO2 methanation (CO2 + 4H2 ⇆ CH4 + 2H2O, ΔĤ °298 K = −165 kJ/mol, also named the Sabatier reaction):3−6 (i) the dissociation of CO2 to CO (or carbonyl (COad)) via formation of carboxylate (HOCO) species prior to methanation, and the subsequent reaction follows the same route as CO methanation (CO + 3H2 ⇆ CH4 + H2O, ΔĤ °298 K = −206 kJ/mol), and (ii) the direct associative adsorption of CO2 and H2 to form CH4 without the formation of CO (or COad) as an intermediate, but it forms formate (HCOO) species. The former proposed methanation mechanism was named a dissociative scheme, while the latter was called an associative scheme (refer to Scheme S1).7 Similarly, for CO methanation, two types of © 2017 American Chemical Society
methanation mechanisms were proposed, the dissociative scheme involves surface-carbon (Cad) as intermediate, while the CHxOad intermediate was present in the associative scheme.6 It is well-known that Pt tends to selectively produce CO from CO2/H2 mixture via the reverse water−gas shift reaction (RWGS, CO2 + H2 ⇆ CO + H2O, ΔĤ o298 K = 41.2 kJ/mol).8,9 Therefore, very few studies use Pt catalysts alone for CO2 hydrogenation toward CH4, where the formation of CH4 in some reports is probably related to synergetic effects of reducible oxide carriers and the Pt crystals.10,11 Even for the bimetallic Co@Pt core−shell catalyst, CO is produced almost exclusively (selectivity to CH4 was Au0.5Cu0.5 (16.0%) > Cu (8.5%) > Au (6.0%) > Ag (4.4%). Regarding product distribution, under all the tested conditions, CO and CH4 were the only two products found (no other higher molecular hydrocarbons (C2−C4) were detected); it appears that more CH4 formed at higher conversion of CO2. The temperature-dependent performance of different catalysts was investigated over 260−320 °C at 1 bar. Arrhenius-type expressions were employed for calculation of apparent activation energies. The linear Arrhenius-type plots of different catalysts are given in Figure 6b, which gives the following order of apparent activation energies for CO2 hydrogenation: Au (62.6 kJ/mol) < Pt (71.8 kJ/mol) < Au0.5Cu0.5 (76.2 kJ/mol) < Cu (81.4 kJ/mol) < Ag (84.8 kJ/mol) < Pt0.5Cu0.5 (90.4 kJ/
mol). These values are all close to those reported from RWGS reaction over noble metal catalysts (in the range of 50−110 kJ/ mol),43 indicating that the formation of CO is the initial step (conversion rate 93.3%) or major CH4 (>92.6%) by adjusting the reaction operation parameters. The used catalysts were further examined by TEM, XRD, and N2 physisorption (Figures S27−S29), showing that the catalysts maintained their morphological and structural integrities and Pt nanoparticles were against sintering due to the physical barriers of three dimensionally assembled cobalt silicate nanosheets confined inside the mazelike nanoreactors. Issue of Diffusion Pathways in Nanocatalysts. From the experimental observations, it is concluded that the asdesigned Pt@CSN (CSN = cobalt silicate nanocubes) nanoreactors are highly active for catalyzing CO2 hydrogenation (methanation). By tuning the retention time and catalyst structures, one can deduce how the intermediate product can be converted to the final product. Our results also claim that the CO2 methanation involves two steps: (i) CO formation occurs over Pt site through the reverse water−gas shift (RWGS) reaction and (ii) sequential CO methanation lead to CH4 formation at the transition metals (Co or Mn). As compared to other catalysts, Pt@CSN nanoreactors seem to have a better configuration for tandem catalysts involving cascade reactions. In terms of the orders of the two different active sites in the system, active sites for the initial reaction reside in the core and active sites for the sequential reaction stay in the shell, which could considerably improve the yield of target products due to the prolonged retention time of intermediate products. Therefore, Pt@CSN is superior to CoSi@Pt due to the existing mesoporous cobalt silicate shell serving as an additional diffusion pathway which not only facilitates the adsorption of reactant gases (i.e., larger surface area) but also guarantees the sufficient retention time for the formation of CH4 from CO. It should be noted that the 7517
DOI: 10.1021/acscatal.7b01827 ACS Catal. 2017, 7, 7509−7519
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molecular sizes of H2 and CO2 are 0.29 and 0.33 nm, respectively, which are much smaller than the average pore size in the Pt@CSN sample (2.6 nm). Therefore, the nanoscale diffusion pathways in this study will not cause the movement limitations of gases, but they will create longer courses for gases across the catalyst bed (see the cartoon illustration in Scheme 1b and model iv in Figure 1d). In general, the total traveling pathways in catalyst nano/microstructures are the sum of intervening space, pores, channels, the encapsulation shells, and so on. Although these explanations are oversimplified in the interest of brevity, they are important for understanding the performance of nanostructured catalysts in a traditional macroscopic fixed bed reactor particularly involving cascade reactions.
CONCLUSIONS In summary, we have proposed a general route for the synthesis of a family of ZIF-67-derived nanoreactors via a water-soakingassisted phase-transformation method for gas-phase CO2 hydrogenation, which allow the facile tuning of product selectivity. In our designed bifunctional nanoreactors, CO2 hydrogenation involves two cascade reaction steps: (i) CO formation via reverse water−gas shift (RWGS) over Pt active site, and (ii) CO methanation to CH4 over the transition-metal active site (Co or Mn). Interestingly, the stacking of ultrathin layer-structured cobalt silicate in the shell not only provides large surface area (700−800 m2/g) for gas adsorption and metal nanoparticle immobilization but also offers additional diffusion pathways for gases/reaction intermediates on catalyst surface, which essentially increases the retention (or trapping) time and enhances the probability for CO to further convert to CH4. We envision that this methodology will open up a new avenue for exploring reaction mechanism through design of catalyst structures. ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01827. Other synthetic procedures and additional experimental results of the studied samples, including Figures S1−S29 and Scheme S1 (PDF)
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Research Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Guowu Zhan: 0000-0002-6337-3758 Hua Chun Zeng: 0000-0002-0215-7760 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, and the National University of Singapore (NUS). G.Z. thanks the NUS Graduate School of Integrative Sciences and Engineering (NGS) for providing his Ph.D. scholarship. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program. 7518
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